The present invention relates to the coating of surfaces. In particular, the invention concerns a method for coating the inner surfaces of equipment with a layer of material.
The purpose of coating surfaces is to improve or alter the properties of a material, such as the resistance of corrosion and stress, optical or electrical properties or to reduce friction. The material of the coating is selected by the application it is used for and by the material that is to be coated. The coatings may be metals or ceramics depending on the desired property and on the operating conditions. The motive for coating pipes and the inner surfaces of tanks is most often to improve resistance to corrosion (both chemical and abrasive corrosion) and, occasionally, to reduce friction.
Recently, new methods for coating the inner surfaces of pipes have been developed. Of the physical methods (PVD), may be mentioned ion beam sputtering, in which method a conical target material is moved inside the pipe and a sputtering ion beam is directed into the pipe from the other end of the pipe (W. Ensinger: Surface and Coatings Technology, 86/87 (1996) 438; A. Schumacher, G. Frech, G. K. Wolf: Surface and Coatings Technology, 89 (1997) 258). The method has been applied only to growing some metal and nitride films, and the dimensions of the pipe that is coated, including its length and diameter, have been only in the order of centimeters.
The other PVD-method is based on the use of plasma in the coating process (Surfcoat). With this method it is at this moment possible to coat pipes that have a diameter of 30 mm and a length of 1000 mm. The quality of the coating is approximately similar to the quality of normal plasma coating. Evaporation is one of the most common PVD-techniques.
The defect of all the PVD-methods is the limited size of the pipe that can be coated. The bending places are still a clear problem and the quality of the film is, even at its best, only of the quality that can be achieved on a plane substrate.
The inner surfaces of the pipes can also be coated electrochemically, especially with electroless plating (auto catalyst) technique. According to the method, the metal is reduced from solution chemically. This technique can be applied only to certain materials (metals and certain compounds). The advantage of this method is that the conformality may be good, as is evidenced by an example of a Cu coating with USLI-technology (V. M. Dubin et al. Journal of the Electrochemical Society 144 (1997) 898).
The chemical vapour deposition (CVD) is a known method for growing conformal thin films. Satisfactory results are obtained, when the chemical reaction functions as desired. In prior art CVD is also suggested to be used in coating the inner surfaces of pipe (L. Poirtier et al., Electrochemical Society Proceedings 97-25 (1997) 425). In general, the studied solutions comprise the coating of metal pipes with a ceramic coating and the lengths of the pipes have been in the order of a few centimeters. A known example of using CVD technique for coating inner surfaces of pipes is the manufacture of the inmost layer of the fiber, which is made by growing a layer inside a billet tube, in the manufacture of optical fiber. According to CVD method the reactant flowing through the tube is attached to the surface of the tube by heating a narrow area at a time. The hot area is thus moved forward along the tube while the tube is rotated. After growing a layer, the tube is collapsed and, thereafter, the actual pulling of the fiber can take place (T. Li: Optical Fiber Communications, part 1, Fiber fabrication, Academic Press, Orlando 1985, p. 363).
The defect of the methods described above is their lack of possibility to coat atomically accurate complicated (bended), large pipings or vessels. Likewise, each method is appropriate only for producing a film with certain constitution.
The atomically controlled production of material is known as Atomic Layer Epitaxy (ALE) method, U.S. Pat. No. 4,085,430. The production of material according to the method is performed by placing the body to be coated in a reactor where conditions enabling alternating surface reactions between the body to be coated and each necessary gaseous reagent are created [T. Suntola: Thin Solid Films 216 (1992) 84]. Typical bodies to be coated are wafers and glass substrates for the manufacture of, among other things, flat displays.
The size and shape of the ALE-reactor determine typically the size and shape of the bodies that can be coated. Since in most of the reaction solutions protective gas is used for carrying the reagents and for separating individual reaction steps, the shape of the body to be coated should be such that enables a sufficiently homogenous gas flow in the reactor.
The objective of the present invention is to remove the problems of the prior art and to provide an entirely new solution by using alternating surface reactions.
The invention is based on the idea that the inner surface of the equipment is coated by making the inner space of the body a closed, controlled gas space, the gas content of which is controlled with valve gears that are used for closing the inner space of the body. With the help of valve gears the interior of the body is alternately filled with the reagent gases required, the partial pressures of which are sufficient to saturate the reactive points of the surface. In other words, the amount of the gas molecules is as great as, or greater than the amount of the reactive sites. Thus, in each stage the reagent that is fed into the space forms an atomic layer of the material donated by the reagent onto the inner surface of the body. The density of the atomic layer is determined by the density of the reactive sites. The temperature of the inner surfaces of the body is controlled with the help of heating device placed outside the body or by feeding heat-transfer liquid or gas into the body before the coating step.
More specifically, the process according to the invention is characterised by what is stated in the characterising part of claim 1.
Considerable advantages are obtained with the aid of the invention. The method is particularly practical for coating the inner surfaces of pipes and piping and different kinds of tanks and facilities that consist of both pipes and tanks. In this invention the ALE-method is used for coating the inner surfaces of pipes and tanks without using separate growing equipment. For this reason, the size of the surface to be coated is not limited, but the method can be used to coat entire process configuration or even the whole piping of a factory. Furthermore, with present invention, the problematic areas, such as the angle parts, can be well coated. Similarly, the films can be grown on non-conductors, for which the electrical methods are known to be inapplicable.
The characteristic features and the advantages of the invention shall become apparent from the following detailed description. In the description, enclosed drawings are referred to. Of these drawings.